The basic structure of an electrochemical cell generally includes electrodes, an anode and a cathode arranged in opposition to one another within a compartment-like cell box. The cell box can contain one or more electrolytes, generally termed anolyte and catholyte depending upon which electrode happens to be in contact with the particular electrolyte.
Often for reasons of electrical efficiency, product purity, or other reasons, such cells will include a separator between the anode and cathode. The separator functions to separate the electrolytes, and may be either porous or non porous. Generally where the separator is non porous, such a separator will be possessed of ion exchange capability so that electrical current can be transferred between the electrodes through the separator. Conventionally, porous separators are termed diaphragms, and non porous separators are termed membranes.
Traditionally, electrodes within such cells have been configured as plate-like surfaces or plate-like mesh surfaces opposing one another to present a desirably large surface area in nearly direct (flow of electrical current being at right angles to the surfaces) opposition to at least one other electrode within the cell. Where such electrodes have been used with porous separators, it has often been necessary to space the electrode from the separator to avoid overvoltages associated with portions of the separator blinding surfaces of the electrode and thereby interfering with the releasing of gas bubbles being evolved at the electrode. Where such electrodes have been used with non porous separators, it has often been desirable to space the separator from the electrode to avoid mechanical damage to often fragile membranes. Such a spacing functions to increase the distance between electrodes within a cell, and thereby increases the electrical potential or voltage required to support cell operation. Operation at an elevated voltage increases the electrical power required to support cell operation placing such a cell operation at an economic disadvantage.
A number of proposals have been directed at improving the power consumption economics of electrochemical cells through decreases in the spacing between anode and cathode within an electrochemical cell. One such improvement has been the introduction of non porous membranes into such cells; generally such membranes can be operated at a closer anode cathode spacing than can diaphragms in the same cell. These membranes are frequently based upon a copolymeric perfluorocarbon material possessed of ion exchange capability. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. I. duPont under the name Nafion.RTM..
These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers usually containing an SO.sub.2 F, that is a sulfonyl fluoride group, or a group including or derived from COF, that is carbonyl fluoride. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F or CF.sub.2 .dbd.CFR.sub.1 COF. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising generally 1 to 8 carbon atoms but occasionally as many as 25 carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F or COF, particularly where the functional group exists as the --(--SO.sub.2 NH).sub.m Q form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups. Typical methyl carboxylate containing monomers are set forth in U.S. Pat. No. 4,349,422.
Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer and/or methyl carboxylate monomers such as perfluoro(4,7-dioxa-5-methyl-8 nonenoate) have found particular acceptance in Cl.sub.2 cells.
In chlorine cells using a sodium chloride brine feedstock, one drawback to the use of perfluorocarbon separators having pendant sulfonyl fluoride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic including OH.sup.- radicals from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce oxygen. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were provided as pendant carboxylate groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135 and 4,349,422.
Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many batteries, fuel cells and electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating these electrodes, remotely positioned on opposite sides of the cell separator.
Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. In some of these SPE proposals, at least one of the electrodes has been a composite of a perfluorocarbon polymer such as Teflon.RTM., E. I. duPont polytetrafluoroethylene (PTFE), with a finely divided electrocatalytic anode material or a finely divided cathode material. In others, the SPE is sandwiched between two such polymer containing electrodes. Typical sandwich SPE cells using non-polymer containing electrode are described in U.S. Pat. Nos. 4,144,301; 4,057,479; 4,056,452 and 4,039,409. SPE composite electrode cells including at least one polymer containing electrode are described in U.S. Pat. Nos. 3,297,484; 4,212,714 and 4,214,958 and in Great Britain Patent Application Nos. 2,009,788A; 2,009,792A and 2,009,795A.
Use of the composite electrodes can significantly enhance cell electrical power efficiency. However, drawbacks associated with present composite electrode configurations have complicated realization of full efficiency benefits. Composite electrodes generally are formed from blends of particulate PTFE and a metal particulate or particulate electrocatalytic compound. The PTFE blend is generally sintered into a decal-like patch that is then applied to a perfluorocarbon membrane. Heat and pressure are applied to the decal and membrane to obtain coadherence between them. A heating process generating heat sufficient to soften the PTFE for adherence to the sheet can present a risk of heat damage to cationic exchange properties of the membrane.
These PTFE based composites demonstrate significant hydrophobic properties that can inhibit the rate of transfer of cell chemistry through the composite to and from the electrically active component of the composite. Therefore, PTFE content of such electrodes must be limited. Formation of a porous composite has been proposed to ameliorate the generally hydrophobic nature of the PTFE composite electrodes, but simple porosity has not been sufficient to provide results potentially available when using a hydrophylic polymer in constructing the composite electrode.
It has been found, at least for use in chlor-alkali cells, that perfluorocarbon copolymer used for forming a membrane should be of an equivalent weight of between at least about 900 and about 1500 to provide a membrane with desirable performance characteristics. Membranes of lower equivalent weight have been found excessively susceptible to chlor alkali cell chemistry, while those of an equivalent weight beyond 1500 have been found insufficiently cation permeable to provide an attractive low resistance cell membrane. To date efforts to utilize a hydrophylic perfluorocarbon copolymer such as NAFION have been largely discouraged by difficulty in forming a commercially acceptable composite electrode utilizing these copolymeric materials. While presently composites are formed by sintering particles of PTFE until the particles coadhere, it has been found that similar sintering of perfluorocarbon copolymers having pendant cation exchange functional activity can significantly dilute the desirable cationic exchange performance characteristics of the copolymer in resulting composite electrodes.
An analogous difficulty has surfaced in the preparation of SPE sandwiches employing more conventional electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally rectilinear electrode into one surface of a perfluorocarbon copolymeric membrane. In some instances, a second similar electrode is simultaneously or subsequently pressed into the obverse membrane surface. To avoid heat damage to the perfluorocarbon membrane, considerable pressure, often as high as 6000 psi is required to embed the electrode firmly in the membrane. Depending upon the configuration of the embedded electrode material, such pressure is often required to be applied simultaneously over the entire electrode area, requiring a press of considerable proportions when preparing a commercial scale SPE electrode.
Often where a foraminous electrode such as a mesh of titanium coated with a chlorine release electrocatalyst or a nickel mesh contacts a membrane in a cell, gases released at the electrode adhere to portions of the membrane causing a blinding effect thereby restricting cation passage therethrough. This restriction elevates the electrical voltage required for cell operation, and thereby effectively increases operational power costs.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of perfluorocarbon composite electrodes and for solvent welding the composites to perfluorocarbon membranes where the perfluorocarbons are of relatively elevated equivalent weights desirable in, for example, chlorine cells, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for dispersing or solvating in part these higher equivalent weight perfluorocarbons.
Where efforts to solvate perfluorocarbon copolymer of desirably elevated equivalent weight has been moderately successful, and where the solvated perfluorocarbon copolymer has been used for forming an electrode including a particulate electrocatalyst, it has been found that the solvated perfluorocarbon can blind the electrocatalyst particles after formation of the electrode and reduce their catalytic activity. Since these electrocatalysts are often compounds of quite expensive metals such as the platinum group metals of ruthenium, iridium, osmium, palladium, rhodium, and platinum, blinding necessarily leads to the inclusion of additional compensatory quantities of the electrocatalyst in the electrode, an undesirable expense.